The best replacement for antigens is antibodies.

The best alternatives to antigens are rated as following:
★★★★★  antibodies
★  immunogens
★  pathogens
★  agglutinogens
★  antibody
★  antibodies blood
  allergens
  antibodies difference

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Speaking about antigens, one probably shouldn't forget about blood type.

Other search terms related to antigens and their ratings are:
★★★★★ blood type
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antigens
antibodies
immunogens